This invention is in the field of semiconductor integrated circuits. Embodiments are more specifically directed to programmable memory locations of the fusible link type in such integrated circuits.
Many modern integrated circuits, particularly those that incorporate multiple functional components of a computer system for controlling and managing a wide range of functions and useful applications, include some type of non-volatile memory resources for storing information. This non-volatile memory may vary in size from a single register location, for example a small “trim” register for setting a particular analog level or setting for the integrated circuit, to one or more configuration registers for customizing the function of the integrated circuit, to an array of hundreds of kilobytes or more of programmable read-only memory (PROM) for storing the device firmware or other program code. These non-volatile memories permit the device manufacturer or system integrator to adjust and configure the functionality of the integrated circuit.
Semiconductor fuse elements, or fusible links, have been widely used for years to implement smaller programmable non-volatile memories such as trim registers and configuration registers. Semiconductor fuses are also commonly used to selectively enable “redundant” memory cells to replace defective cells in larger memory arrays, and even for storing small blocks of program code. Conventional fusible links are typically constructed as a small conductive element, for example of polycrystalline silicon or a metal conductor, often having a small necked-down portion of smaller cross-sectional area than the remainder of the conductor. “Blowing” or programming of the fusible link is typically carried out by applying current of a sufficient magnitude to cause a physical change in the structure of the fusible link, for example by melting the material to open the link. Another type of fusible link is constructed so that the programming current instead degrades the fuse structure to significantly decrease its resistance, without opening the link. So-called “antifuses” are fusible links that are normally non-conductive but are “blown” closed when programmed. Laser-programmable fuses are fusible links that are blown open by externally-applied laser energy, and as such are typically programmed by the device manufacturer. For any of these types, once the fusible link is programmed, its state is non-volatile, in that the link cannot be again electrically closed or otherwise returned to its original state.
In modern integrated circuits realized using current-day technology, it has been observed that the chip area required for the peripheral circuitry involved in electrically programming and sensing the state of a fusible link dwarfs the chip area consumed by the fusible link itself. For example, in one conventional device, the fusible link itself occupies on the order of 1.5% of the chip area required for the peripheral programming and sense circuitry for that single fusible link. Accordingly, the implementation of even a modest register location using fusible links can require significant chip area, on the order of 70× the chip area of the links themselves.
By way of further background, conventional register locations involving fusible links are often implemented as a “fuse chain”, with the peripheral circuitry for the links in that chain serially connected to reduce the number of conductors required to communicate data to and from the register. Serial data communication to and from wide registers, such as 64-bit or larger registers, thus necessarily results in significant latency and very slow bit transmission rates. For example, the serial communication of 64 bits at a clock cycle time of about 300 nsec requires nearly 20 μsec, which is extremely slow as compared with the computational rates of modern high-speed integrated circuits.